1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2009-53271 describes a method for manufacturing a distributed feedback laser diode by using a nano-imprint technique. In this method, a semiconductor layer is patterned by the nano-imprint technique to form a diffraction grating of a distributed feedback laser diode. Also, a microfabrication method in which a nano-imprint technique is used is described in M. W. Lin, et. al., “Planarization for Reverse-Tone Step and Flash Imprint Lithograph”, Proceedings of SPIE 6151, pp. 61512G (2006).
The nano-imprint technique has been studied to apply it to a semiconductor process of forming a diffraction grating in a semiconductor laser. The manufacturing cost of devices such as semiconductor lasers can be reduced by forming the diffraction grating by the nano-imprint technique.
To form a diffraction grating in a semiconductor laser by the nano-imprint technique, first, a semiconductor layer on which the diffraction grating is to be formed and a resin part are formed on a semiconductor substrate in that order. Then, a mold having a pattern surface is pressed against the resin part. A pattern of the diffraction grating having periodic projections and recesses is formed on the pattern surface of the mold. Next, the resin part is hardened in that state. Subsequently, the mold and the resin part are detached from each other. Thus, the pattern of the diffraction grating formed on the pattern surface of the mold is transferred onto the resin part, so that a resin pattern portion is formed on the resin part.
Subsequently, the semiconductor layer is etched by using the resin pattern portion, so that the pattern formed in the resin pattern portion is transferred onto the semiconductor layer. Thus, the diffraction grating is formed on the semiconductor layer.
However, in the above-described method for manufacturing a diffraction grating by the nano-imprint technique, air may remain between the resin part and the recesses in the pattern surface of the mold when the pattern surface is pressed against the resin layer. Consequently, gas bubbles may become trapped between the resin part and the recesses in the pattern surface. If the resin part is hardened in such a state, the shape of the recesses in the mold cannot be appropriately transferred onto the resin part. As a result, defects are easily formed in the diffraction grating on the semiconductor layer.